The invention relates generally to a method for estimation of global error motion vectors, which represent unwanted global picture instabilities in a picture sequence in digital video signals. The method uses a number of selected reference windows as well as spatial processing (involving calculations based on information associated with one picture in the picture sequence), motion vector estimators (estimating global motion vectors or vector fields associated with complete pictures or local motion vectors associated with fractions of pictures, whereat, preferrably, a confidence value indicative of reliability of the estimation is assigned to every motion vector) using a specific measuring time distance (corresponding to the time elapsed between two pictures in the picture sequence, which are to be compared with regard to individual pixels in order to estimate inter-picture displacements yielding global or local motion vectors for one of the two pictures), as well as temporal processing (involving calculations based on information associated with several pictures in the picture sequence).
This is done in order to estimate a sequence of global motion vectors (represented by one two-dimensional motion vector per picture), from which a sequence of global error motion vectors (associated with the unwanted global picture instabilities and represented by one two-dimensional motion vector per picture) is separated. The sequence of global error motion vectors may be used for compensation of the unwanted picture instabilities to allow production of stabilized picture sequences. Due to e.g. bending and stretch of film originals the global motion vector may also be corrected locally within a picture in order to minimize picture distortion.
In general, motion in a picture sequence is a combination of object motion (associated with moving objects) and global motion. The global motion is a combination of true global motion (such as pan and zoom) and unwanted global picture instabilities resulting from, e.g., worn-out film sprocket holes, poorly performing telecine (device used in transfer of film to video), linear or non-linear stretch of film original during printing or copying, or unsteady camera shots. The true global motion, and the unwanted global picture instabilities may vary in amplitude and frequency. The variation in the amplitude may range from small to large (compared to picture spatial resolution) and the variation in the frequency may range from low to high (compared to picture temporal resolution), within the picture sequence. In the case of film stretch and bend the true global motion may differ within one picture, and thus the unwanted global motion will be described by a global error motion vector field for the entire picture.
The global motion may be represented by a sequence of global motion vectors, wherein the amplitude corresponds to a maximum length of the global motion vectors within a specific interval of the picture sequence. The maximum length may range from fractions of an inter-pixel distance to several times this distance, where large amplitudes correspond to large maximum length and small amplitude correspond to small maximum length. The frequency corresponds to the global motion vector frequency, i.e. the frequency content of the sequence of global motion vectors within the specific interval of the picture sequence, the global motion vector frequency ranging approximately from picture frequency resolution limit to fractions thereof.
Known methods for estimating global error motion vectors involve motion vector estimators using known motion vector estimation techniques such as, e.g., pixel gradient recursion, phase correlation, and block matching (see, e.g., PCT/SE92/00219), combined with spatial and temporal processing of the estimated motion vectors.
One difficulty in the motion vector estimators is to make accurate estimation of both large and small motion vectors. This is due to the fact that these motion vector estimators use a fixed measuring time distance, and that a small measuring distance is advantageous for the estimation of large motion vectors, whereas a large measuring distance is advantageous for the estimation of small motion vectors at low motion vector frequencies. The latter is a consequence of that the finest possible estimation of motion vectors is limited by picture resolution for a fixed measuring distance, provided that the motion vector frequency is low. These motion vector estimators thus give a compromise between the two cases, leading to limitations in accurate estimation of small global motion vectors at low motion vector frequences, which represent a type of global picture instabilities particularly critical to human visual reception, especially when mixing two or more picture sequences.
One difficulty in the spatial and temporal processing is to make a precise estimate of a global motion vector from estimated motion vectors associated with a picture in the picture sequence.
Another difficulty in spatial and temporal processing is to make a precise estimate of a global error motion vector from estimated the global motion vectors associated with a picture in the picture sequence. The precision of the estimate of the global error motion vector is dependent upon a filtering method used to separate global error motion vectors from global motion vectors associated with pictures in the picture sequence.